You might start out small. Install BuddyPress and enable just the activity stream; disable everything else. Make sure that the theme css works well with the activity stream. You’ll need to install the BuddyPress template pack. After you are satisfied with the activity stream then change WP install to multi-site. You’ll want to lock it down so that you control creation of blogs – so that you don’t have to deal with splogs. Once you have that working then you can add a few other BP features – private messaging, etc.